A) Field of the Invention
The present invention relates to a ZnO based semiconductor light emitting device and its manufacture method.
B) Description of the Related Art
A transition energy between bands of zinc oxide (ZnO) corresponds to an emission energy in a ultraviolet range of about 370 nm. This wavelength is shorter than a visual light range of 400 nm or longer providing a high industrial usage value. In order to obtain emission at a wavelength longer than 400 nm, studies have been made to narrow a band gap (gap narrowing) of ZnO.
As the gap narrowing techniques of narrowing a band gap of ZnO, ZnCdO mixed crystal replacing a portion of Zn with Cd has been proposed in which the band gap can be adjusted in a range of 3.4 eV to 1.5 eV depending on a composition of Cd. However, since Cd is a highly toxic element, adopting ZnCdO mixed crystal is difficult from the viewpoint of safety.
For example, Japanese Patent Laid-open Publication No. 2002-16285 discloses techniques of gap narrowing of a band gap of ZnO by replacing a portion of O with a group VI element of sulfur (S) or selenium (Se) and forming mixed crystal. The techniques are excellent from the viewpoint of safety, as compared to the techniques of introducing Cd.
Japanese Patent Laid-open Publication No. 2002-16285 also discloses a light emitting device by sandwiching a gap-narrowed ZnO based compound semiconductor layer (ZnOS layer, ZnOSe layer) between ZnMgO clad layers.
According to B. K. Mayer et al: Appl. Phys. Lett. 85 (2004) 4929, a relation between an S composition x and a band gap of ZnO1-xSx (0≦x≦1) is represented by:EZnOS=xEZnS+(1−x)EZnO−b(1−x)x where EZnOS, EZnS and EZnO are band gaps of ZnO1-xSx, ZnS and ZnO, respectively, and b is a bowing parameter. It is said that a band gap of ZnO1-xSx can be narrowed to 2.6 eV. The bowing parameter b is 3 eV.
Techniques have been desired which can be applied to improving an emission efficiency of a ZnO based semiconductor light emitting device using a ZnO1-xSx emission layer.